A typical field emission device contains electron emitters, such as Spindt tips, which are made from an electron-emissive metal, such as molybdenum. These electron emitters are susceptible to surface contamination by oxygen-containing and carbon-containing species. The surface oxygen and carbon have deleterious effects on the electron emission properties of the electron emitters. In particular, the presence of oxygen and carbon at the emissive surface increases the surface work function of the electron emitters. That is, a bigger electric field is required to extract electrons therefrom due to the contamination. Surface contaminants also result in emission current instability and reduced device lifetime.
Metal field emission tips have been employed in field emission electron and ion microscopy. It is known to remove surface contaminants from electron emitters in these microscopy devices by employing high temperature (greater than 2000.degree. K.) flashing. However, field emission arrays often include glass substrates upon which the electron emitters are formed. These glass substrates have temperature tolerances upwards of 700.degree.-800.degree. K. Thus, high temperature cleaning procedures cannot be used for decontaminating field emission electron emitters formed on glass substrates.
Furthermore, the contamination of field emission electron emitters occurs throughout the life of the field emission device. Contaminant gaseous species are introduced into the vacuum of the field emission device by outgassing from surfaces, by electron-stimulated desorption from the phosphors and other surfaces that are exposed to field emitted electrons, by small leaks in the packaging elements, etc.
In order to maintain constant emission characteristics over the life of the device, it is desirable that emitter surface contaminants be removed throughout the life of the device. It is also desirable that this cleaning process be continuous over the life of the device or be performed periodically at a frequency that is sufficient to prevent appreciable deterioration of emission characteristics. However, field emission devices typically have no convenient means for introducing cleaning agents into the device subsequent to the vacuum sealing of the device package.
Accordingly, there exists a need for a field emission device having means for in situ removal of surface contaminants from field emission electron emitters.